In the semiconductor industry, disc-shaped semiconductor wafers form the basic medium for manufacturing electronic integrated circuits and devices. Typically, each wafer is subjected to a series of processing steps necessary to form tens of thousands of microelectronic devices. Thus, it becomes apparent that such wafers acquire considerable value at each processing step is performed. Since these wafers are fragile, and their features are delicate, the processing steps usually take place in a clean room environment to avoid particulate contamination of the wafers. The transport of the wafers in and out of such clean room, and between processing stations within the clean room, is typically achieved by using wafer carriers or cassettes capable of accommodating about 25 wafers, with each wafer held in a separate pocket of the cassette in a substantially upright position.
In an effort to minimize damage and particulate contamination of the wafers and thereby achieve high manufacturing yields, operator handling of the wafers is being substantially avoided by resorting to automation. In an automated semiconductor wafer processing facility, it becomes essential to first identify each individual wafer of a cassette to verify that the correct wafers are in such cassette. Also, the identification of each wafer is required to insure that the appropriate processing steps associated with a particular wafer are indeed carried out on such wafer.
Various forms of wafer identifications have been used in the manufacturing of semiconductor integrated circuits. For example, conventional letters and numerals have been placed on the surface of a wafer so that a technician could look at the wafer and determine its type and category without requiring a detailed wafer pattern analysis. Clearly, such a known technique does not avoid operator handling of the wafers. Moreover, such a technique substantially reduces the throughput of the integrated circuits production line.
Several attempts have been made at automatic identification of semiconductor wafers. One technique which has been suggested is the etching of the back surface of a wafer to form a code. Light is directed at the back surface of the wafer and the radiation reflected therefrom is detected to provide an electrical indication of the code inscribed in the wafer. The handling of such a wafer to read its back surface may damage the integrated circuits formed on its front surface. Another known identification technique makes use of an etched binary or frequency code in the kerf areas of the front surface of the wafer, i.e., in those unused portions of the front surface between each semiconductor chip of an array of chips formed in the wafer.
Although the foregoing approaches appear to avoid operator handling of the wafers, the accuracy and reliability of such prior art techniques have not proven to be acceptable. In other words, there still exists a need for a reliable, inexpensive and accurate technique for marking an identifying semiconductor wafers.